The invention relates to a marine propulsion unit having a depending gearcase with water inlet openings on the sides for supplying water to a water pump, and an auxiliary water inlet opening at an anti-ventilation plate for supplying additional water to the water pump.
It is standard to provide water inlets on the sides of a depending gearcase of a marine propulsion unit. These side water inlets function well even with the engine mounted at high transom heights, including when the anti-ventilation plate over the propeller is above the water line. However, such side inlets are subject to clogging or plugging at low speeds if weeds or debris wrap around the gearcase strut. It is known in the prior art to provide an auxiliary water inlet opening under the anti-ventilation plate to solve the weed clogging problem. The propeller is spinning beneath such auxiliary water inlet openings and keeps such opening clean and free of weeds and debris, whereby sufficient water is supplied through such opening to the water pump. However, when running at high speed with the engine raised higher out of the water, or at extreme trim angles, such auxiliary water inlet opening may be raised above the water line. This in turn enables air to be sucked in to the water pump, even though the standard side water inlets are submerged. The sucked-in air reduces the amount of cooling water which can be supplied by the pump, and may cause overheating of the engine.
The present invention provides an air trap in the water passage from the auxiliary water inlet opening to the water pump even when the auxiliary water inlet opening is exposed to air, provided the side water inlet openings are submerged.